1. Field of the Invention
The present invention relates to a local area network communication systems suitable for use in combination of time-division multiplexing and frequency-division multiplexing in a multiple-channel broadband communications environment.
Many offices today use a network for transmission of data between personal computers, terminals and other devices within the office. Several types of architectures are used. In a star network, all nodes are coupled to a central point of the star, which provides centralized control of the flow of data. The central control on such a system can multiplex data in time from different nodes by alternately holding data in a buffer from one or the other transmitting node until a timeslot is available. A central control unit provides the synchronization necessary to insert data into assigned or available timeslots. One limitation of the star network is that the bandwidth available through the switch matrix is limited. Hence, means must be provided for multiple nodes to transmit data to the central control unit within a common broadband frequency channel.
A further system architecture employed for data communication is a token ring network. In a token ring network, a single cable or communication line is passed through each and every node, and a token packet is relayed from one node to the next, with the node desiring to transmit holding onto the token packet until its communication period is terminated. A node cannot transmit unless it has the token, and therefore, only one node is permitted to transmit at any time. This type of time-division multiplexing transmits data in irregular bursts, rather than in assigned time slots. This type of transmission is appropriate for data communications which typically occur in infrequent long bursts. Voice communications, on the other hand, require a substantially continuous connection over an extended period of time.
A still further data communication system architecture often employed for data communication is the Ethernet system. In the Ethernet system, a data node listens to see if the channel is in use before transmitting. While transmitting, the data node listens to determine if the data transmitted is received in the same form. If the received data differs, then it is assumed another node transmitted at the same time, resulting in a collision, and thus in scrambled data. The transmitting node then retransmits a packet at a random time later. Thus, neither central control of the network bandwidth acquisition of time slots nor central timing is needed.
2. Description of the Prior Art
The following patents were uncovered in a search for three-level frequency shift keyed (FSK) modulation systems.
Digital Data Modulator and Digital-to-analog Converter. U.S. Pat. No. 4,686,490. This patent discloses a digital modulator for selectably generating differential phase shift keyed (PSK) data signals and frequency shift keyed (FSK) data signals. A dibit encoder converts scrambled dibit into two phase-shifting control signals. A three-tap finite impulse response filter generates carrier phase signals by splitting each cycle of the selected carrier clock into eight phases, and, for psk operation, provides phase shifting in response to the two phase-shifting control signals. A multiplexer selects one of the phase-shifter high frequency clocks in response to the carrier phase signals. A signal selector converts the selected phase-shifted high frequency clock signal into a pulse width-modulated clock signal. The signal filter provides the PSK/FSK data signal by integrating the pulse width modulated clock signal and also provides band pass filtering to clean up the waveform. The result is reported to be a PSK/FSK data signal with low distortion, reduced transient dc, and decreased sensitivity to temperature and component selection since bit weighting resistors are not required to generate the output waveform. Also disclosed is a digital-to-analog converter/data modulator for generating FSK and PSK data signals and a method for generating frequency-modulated signals.
Processes for Generating Duo-binary FSK, Tamed FSK and TFM Modulations and Modulators Applying Those Processes. U.S. Pat. No. 4,426,710. This patent discloses methods to obtain a duo-binary FSK modulation, wherein the modulating binary signal train has a three state partial response and is fed through a precoding, a transition-type coding, a simplified msk modulation at the carrier frequency, a frequency division by two, and a multiplication by the same signal delayed by one binary element period. In order to obtain a "tamed FSK" modulation, the modulating binary signal train has a five state partial response and is fed through precoding, a transition-type coding, a simplified msk modulation at the carrier frequency, a frequency division by two, a multiplication by the same signal delayed, for one part, by one binary element period and, for another part, by two binary element periods. A tfm modulation is obtained by using the FSK duo-binary generating process. In a tfm modulator, a train of binary signals is applied to a partial response precoding circuit, the output of which is connected to a transition-type coding circuit, the output of which is connected to the input of a simplified msk modulator.
Data Encoding and Decoding Communication System for Three Frequency FSK Modulation and Method Therefor. U.S. Pat. No. 4,425,666. This patent discloses a system for three frequency FSK modulation wherein binary data bits are sequentially encoded into a ternary system of channels designated "1", "0" and "C", so that a data bit is encoded and transmitted in a "C" channel whenever it is the same as the previous data bit and wherein a transmission in the "C" channel is decoded as a continuation of the previous data bit. The data clock is recovered from the data stream at the transition between data bits and repeated data bits are detected as errors.
The following patents were uncovered in reference to a Maximum Likelihood Detection Circuit as referred to in this application:
Carrier Synchronization and Symbol Synchronization in Offset QPSK Burst Communications, U.S. Pat. No. 4,313,205. This patent describes a carrier synchronization scheme based on average phase over a periodic interval and wherein phase synchronization is achieved by reliance on the filtered and demodulated upper sideband of the output of a delay and multiply circuit.
Method for Determining the Optimum Sampling Times for a QPSK or QAM Received Signal, U.S. Pat. No. 4,520,492. This patent describes a detector system wherein the amplitude of two quadrature components of a received signal are coupled and the point of identical amplitude is selected as the point of sampling.